1. Field of the Invention
This invention relates to plasma burners.
2. Description of the Prior Art
Plasma burners are utilised in a series of areas for such purposes as the rapid heating of surfaces, melting of materials, heating of solid and liquid materials and conducting chemical reactions at temperatures above those which can be obtained conveniently by other types of heating. The heating occurs mainly by heat rays from the very hot gas components of the plasma flame which are absorbed on the surface of the body which is desired to be heated.
The application of dissimilar types of plasma burners of dissimilar materials for purposes such as burning and melting and other types of heating is well known. A common feature for all these different types of plasma burners is that for any of the parts which the burner is constructed of, there must be avoided, to a substantial degree, any change in shape during operation as a consequence of melting or vaporising. The reason for avoiding a change is that the electrical characteristics of the burner are thereby altered. This arises in that the parts of the burner which are supplied with heat from the plasma flame are cooled. The heat is often led away from these parts by means of a cooling medium (fluid or gas) which flows in ducts in the heat-stressed part of the burner. Perspiration cooling can also be employed. In order to create cooling, the designers of this type of plasma burner have been forced in part to use packings and materials which do not tolerate heating to high temperatures. If an electrically conducting material forces itself into the burner due to a failure and creates an electrical contact between inner and outer electrodes, often called "dysen" (the jet), the short circuit which occurs will often be sufficient to destroy the burner.
It has not been possible, therefore, to utilise the types of burners which have been known hitherto in intensely aggressive environments and at high temperatures, nor submerged in electrically conducting metal, slag or salt melts.
By virtue of the complex construction of the plasma burners known hitherto and the demands which have been placed on the plasma electrodes in these burners, it has been necessary to avoid contact between parts of the plasma burner and metals, slag and salt melts which are kept at high temperatures. This has meant that the designers have not been able to place the plasma burner in contact with the medium which is to be heated. When the designer has elected to place the plasma burner far from the object which is to be heated, this involves a loss of energy in the transfer of heat from the plasma flame. In those cases where the designer has taken the risk of placing the burner, for example, close to a liquid phase, the result has been an undesired local overheating and with this a consequent undesired vaporising of the material which is to be heated.